Fix lampepfl#7044: Added GADT recovery with fallback to default error.

compiler/src/dotty/tools/dotc/typer/Inferencing.scala

@@ -19,7 +19,7 @@ import collection.mutable
 import scala.annotation.internal.sharable
 import scala.annotation.threadUnsafe
 
-import config.Printers.debug
+import config.Printers.gadts
 
 object Inferencing {
 
@@ -165,14 +165,16 @@ object Inferencing {
   }
 
   def approximateGADT(tp: Type)(implicit ctx: Context): Type = {
-    val map = new IsFullyDefinedAccumulator2
+    val map = new ApproximateGadtAccumulator
     val res = map(tp)
     assert(!map.failed)
-    debug.println(i"approximateGADT( $tp )  = $res  //  {${tp.toString}}")
     res
   }
 
-  private class IsFullyDefinedAccumulator2(implicit ctx: Context) extends TypeMap {
+  /** This class is mostly based on IsFullyDefinedAccumulator.
+    * It tries to approximate the given type based on the available GADT constraints.
+    */
+  private class ApproximateGadtAccumulator(implicit ctx: Context) extends TypeMap {
 
     var failed = false
 
@@ -200,9 +202,7 @@ object Inferencing {
         val sym = tp.symbol
         val res =
           ctx.gadt.approximation(sym, fromBelow = variance < 0)
-
-        debug.println(i"approximated $tp  ~~  $res")
-
+        gadts.println(i"approximated $tp  ~~  $res")
         res
 
       case _: WildcardType | _: ProtoType =>
@@ -213,21 +213,8 @@ object Inferencing {
         mapOver(tp)
     }
 
-    // private class UpperInstantiator(implicit ctx: Context) extends TypeAccumulator[Unit] {
-    //   def apply(x: Unit, tp: Type): Unit = {
-    //     tp match {
-    //       case tvar: TypeVar if !tvar.isInstantiated =>
-    //         instantiate(tvar, fromBelow = false)
-    //       case _ =>
-    //     }
-    //     foldOver(x, tp)
-    //   }
-    // }
-
     def process(tp: Type): Type = {
-      val res = apply(tp)
-      // if (res && toMaximize) new UpperInstantiator().apply((), tp)
-      res
+      apply(tp)
     }
   }
 

compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala

@@ -13,6 +13,7 @@ import Decorators._
 import Uniques._
 import config.Printers.typr
 import util.SourceFile
+import util.Property
 
 import scala.annotation.internal.sharable
 
@@ -684,7 +685,14 @@ object ProtoTypes {
 
   /** Dummy tree to be used as an argument of a FunProto or ViewProto type */
   object dummyTreeOfType {
-    def apply(tp: Type)(implicit src: SourceFile): Tree = untpd.Literal(Constant(null)) withTypeUnchecked tp
+    /*
+     * A property indicating that the given tree was created with dummyTreeOfType.
+     * It is sometimes necessary to detect the dummy trees to avoid unwanted readaptations on them.
+     */
+    val IsDummyTree = new Property.Key[Unit]
+
+    def apply(tp: Type)(implicit src: SourceFile): Tree =
+      (untpd.Literal(Constant(null)) withTypeUnchecked tp).withAttachment(IsDummyTree, ())
     def unapply(tree: untpd.Tree): Option[Type] = tree match {
       case Literal(Constant(null)) => Some(tree.typeOpt)
       case _ => None

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -44,7 +44,6 @@ import transform.TypeUtils._
 import reporting.trace
 import Nullables.{NotNullInfo, given _}
 import NullOpsDecorator._
-import config.Printers.debug
 
 object Typer {
 
@@ -2774,7 +2773,7 @@ class Typer extends Namer
    */
   def adapt(tree: Tree, pt: Type, locked: TypeVars, tryGadtHealing: Boolean = true)(using Context): Tree = {
     val last = Thread.currentThread.getStackTrace()(2).toString;
-    trace/*.force*/(i"adapting (tryGadtHealing=$tryGadtHealing) $tree to $pt\n{callsite: $last}", typr, show = true) {
+    trace(i"adapting (tryGadtHealing=$tryGadtHealing) $tree to $pt\n{callsite: $last}", typr, show = true) {
       record("adapt")
       adapt1(tree, pt, locked, tryGadtHealing)
     }
@@ -2784,7 +2783,6 @@ class Typer extends Namer
     adapt(tree, pt, ctx.typerState.ownedVars)
 
   private def adapt1(tree: Tree, pt: Type, locked: TypeVars, tryGadtHealing: Boolean)(using Context): Tree = {
-    // assert(pt.exists && !pt.isInstanceOf[ExprType])
     assert(pt.exists && !pt.isInstanceOf[ExprType] || ctx.reporter.errorsReported)
     def methodStr = err.refStr(methPart(tree).tpe)
 
@@ -3243,19 +3241,15 @@ class Typer extends Namer
     }
 
     def adaptToSubType(wtp: Type): Tree = {
-      debug.println("adaptToSubType")
-      debug.println("// try converting a constant to the target type")
       // try converting a constant to the target type
       val folded = ConstFold(tree, pt)
       if (folded ne tree)
         return adaptConstant(folded, folded.tpe.asInstanceOf[ConstantType])
 
-      debug.println("// Try to capture wildcards in type")
       val captured = captureWildcards(wtp)
       if (captured `ne` wtp)
         return readapt(tree.cast(captured))
 
-      debug.println("// drop type if prototype is Unit")
       // drop type if prototype is Unit
       if (pt isRef defn.UnitClass) {
         // local adaptation makes sure every adapted tree conforms to its pt
@@ -3265,7 +3259,6 @@ class Typer extends Namer
         return tpd.Block(tree1 :: Nil, Literal(Constant(())))
       }
 
-      debug.println("// convert function literal to SAM closure")
       // convert function literal to SAM closure
       tree match {
         case closure(Nil, id @ Ident(nme.ANON_FUN), _)
@@ -3283,28 +3276,28 @@ class Typer extends Namer
         case _ =>
       }
 
-      debug.println("// try GADT approximation")
-      val foo = Inferencing.approximateGADT(wtp)
-      debug.println(
-        i"""
-        foo = $foo
+      val approximation = Inferencing.approximateGADT(wtp)
+      gadts.println(
+        i"""GADT approximation {
+        approximation = $approximation
         pt.isInstanceOf[SelectionProto] = ${pt.isInstanceOf[SelectionProto]}
         ctx.gadt.nonEmpty = ${ctx.gadt.nonEmpty}
+        ctx.gadt = ${ctx.gadt.debugBoundsDescription}
         pt.isMatchedBy = ${
           if (pt.isInstanceOf[SelectionProto])
-            pt.asInstanceOf[SelectionProto].isMatchedBy(foo).toString
+            pt.asInstanceOf[SelectionProto].isMatchedBy(approximation).toString
           else
             "<not a SelectionProto>"
         }
+        }
         """
       )
       pt match {
-        case pt: SelectionProto if ctx.gadt.nonEmpty && pt.isMatchedBy(foo) =>
-          return tpd.Typed(tree, TypeTree(foo))
+        case pt: SelectionProto if ctx.gadt.nonEmpty && pt.isMatchedBy(approximation) =>
+          return tpd.Typed(tree, TypeTree(approximation))
         case _ => ;
       }
 
-      debug.println("// try an extension method in scope")
       // try an extension method in scope
       pt match {
         case SelectionProto(name, mbrType, _, _) =>
@@ -3322,33 +3315,41 @@ class Typer extends Namer
           val app = tryExtension(using nestedCtx)
           if (!app.isEmpty && !nestedCtx.reporter.hasErrors) {
             nestedCtx.typerState.commit()
-            debug.println("returning ext meth in scope")
             return ExtMethodApply(app)
           }
         case _ =>
       }
 
-      debug.println("// try an implicit conversion")
       // try an implicit conversion
       val prevConstraint = ctx.typerState.constraint
-      def recover(failure: SearchFailureType) =
-        {
-          debug.println("recover")
+      def recover(failure: SearchFailureType) = {
+        def canTryGADTHealing: Boolean = {
+          val isDummy = tree.hasAttachment(dummyTreeOfType.IsDummyTree)
+          tryGadtHealing // allow GADT healing only once to avoid a loop
+          && ctx.gadt.nonEmpty // GADT healing only makes sense if there are GADT constraints present
+          && !isDummy // avoid healing a dummy tree as it can lead to an error in a very specific case
+        }
+
         if (isFullyDefined(wtp, force = ForceDegree.all) &&
             ctx.typerState.constraint.ne(prevConstraint)) readapt(tree)
-        // else if ({
-        //   debug.println(i"tryGadtHealing=$tryGadtHealing && \n\tctx.gadt.nonEmpty=${ctx.gadt.nonEmpty}")
-        //   tryGadtHealing && ctx.gadt.nonEmpty
-        // })
-        //  {
-        //    debug.println("here")
-        //    readapt(
-        //    tree = tpd.Typed(tree, TypeTree(Inferencing.approximateGADT(wtp))),
-        //    shouldTryGadtHealing = false,
-        //    )
-        //  }
-        else err.typeMismatch(tree, pt, failure)
-        }
+        else if (canTryGADTHealing) {
+          // try recovering with a GADT approximation
+          val nestedCtx = ctx.fresh.setNewTyperState()
+          val res =
+            readapt(
+              tree = tpd.Typed(tree, TypeTree(Inferencing.approximateGADT(wtp))),
+              shouldTryGadtHealing = false,
+            )(using nestedCtx)
+          if (!nestedCtx.reporter.hasErrors) {
+            // GADT recovery successful
+            nestedCtx.typerState.commit()
+            res
+          } else {
+            // otherwise fail with the error that would have been reported without the GADT recovery
+            err.typeMismatch(tree, pt, failure)
+          }
+        } else err.typeMismatch(tree, pt, failure)
+      }
       if ctx.mode.is(Mode.ImplicitsEnabled) && tree.typeOpt.isValueType then
         if pt.isRef(defn.AnyValClass) || pt.isRef(defn.ObjectClass) then
           ctx.error(em"the result of an implicit conversion must be more specific than $pt", tree.sourcePos)

tests/neg/boundspropagation.scala

@@ -25,19 +25,6 @@ object test3 {
   }
 }
 
-// Example contributed by Jason.
-object test4 {
-  class Base {
-    type N
-
-    class Tree[-S, -T >: Option[S]]
-
-    def g(x: Any): Tree[_, _ <: Option[N]] = x match {
-      case y: Tree[_, _] => y                         // error -- used to work (because of capture conversion?)
-    }
-  }
-}
-
 class Test5 {
 "": ({ type U = this.type })#U                        // error
 }

tests/pos/boundspropagation.scala

@@ -29,3 +29,16 @@ object test2 {
   }
 }
 */
+
+// Example contributed by Jason.
+object test2 {
+  class Base {
+    type N
+
+    class Tree[-S, -T >: Option[S]]
+
+    def g(x: Any): Tree[_, _ <: Option[N]] = x match {
+      case y: Tree[_, _] => y
+    }
+  }
+}

tests/pos/gadt-infer-ascription.scala

@@ -0,0 +1,15 @@
+// test based on an example code by @Blaisorblade
+object GadtAscription {
+  enum Var[G, A] {
+    case Z[G, A]() extends Var[(A, G), A]
+    case S[G, A, B](x: Var[G, A]) extends Var[(B, G), A]
+  }
+
+  import Var._
+  def evalVar[G, A](x: Var[G, A])(rho: G): A = x match {
+    case _: Z[g, a] =>
+      rho(0)
+    case s: S[g, a, b] =>
+      evalVar(s.x)(rho(1))
+  }
+}

tests/pos/i7044.scala

@@ -0,0 +1,14 @@
+object i7044 {
+  case class Seg[T](pat:Pat[T], body:T)
+
+  trait Pat[T]
+  object Pat {
+    case class Expr()            extends Pat[Int]
+    case class Opt[S](el:Pat[S]) extends Pat[Option[S]]
+  }
+
+  def test[T](s:Seg[T]):Int = s match {
+    case Seg(Pat.Expr(),body)          => body + 1
+    case Seg(Pat.Opt(Pat.Expr()),body) => body.get
+  }
+}